1. Field of the Invention
The present invention relates to feedback control devices, and more particularly, to a feedback control device capable of stably controlling a controlled system by means of a plurality of controllers.
2. Description of the Related Art
There has been known a control device which includes a plurality of controllers with an identical function for simultaneously controlling a controlled system so that the controlled system can be controlled stably over a long term. In terminal equipment of an optical submarine cable network, for example, two controllers with the same function are used to control a single controlled device.
The terminal equipment in an optical submarine cable network adopts WDM (Wavelength Division Multiplexing) technology using AWG (Array Wave-guide Grating) optical multiplexer/demultiplexer so that optical beams with different wavelengths can be multiplexed to be simultaneously transmitted over a single optical fiber and also that multiplexed light can be demultiplexed into optical beams of different wavelengths.
AWG is a device whose transmission wavelengths vary with temperature because of the temperature dependence of the refractive index of silica glass, and the temperature is controlled to select the wavelengths to be multiplexed. Accordingly, stable control of the temperature is of especial importance.
The following describes a conventional feedback control device which is used, for example, in terminal equipment of an optical submarine cable network for the purpose of temperature control.
FIG. 8 is a schematic block diagram of such a conventional feedback control device.
The feedback control device 500 includes two controllers 510 and 520 for controlling the temperature of a controlled device 600 (e.g., a WDM device with AWG) to a fixed temperature. The controllers 510 and 520 have an identical function and include control data generators 511 and 521, respectively, for generating control data for stably controlling a heater 601 by feedback control, and drivers 512 and 522, respectively, for supplying electric current to the heater 601 in accordance with the control data, to drive the heater 601.
The control data generator 511, 521 comprises a subtractor 511a, 521a for calculating a deviation of the temperature measured by a sensor 602 of the controlled device 600 from a set temperature stored in a set temperature table 603 of the controlled device 600, and an integrating circuit 511b, 521b for generating control data by integrating the deviation.
The driver 512, 522 comprises a control-drive circuit 512a, 522a and a FET (Field-Effect Transistor) 512b, 522b. Based on the control data supplied from the control data generator 511, 521, the control-drive circuit 512a, 522a controls PWM (Pulse Width Modulation) for adjusting the time for which the FET 512b, 522b is switched on or off, to thereby control the current supplied from a power supply VCC to the heater 601.
During normal operation of the conventional feedback control device 500, the controllers 510 and 520 simultaneously perform feedback control to stably control the heater 601. If the heater 601 is controlled in such a manner that only one controller is operated during normal operation and that in case of failure, the other controller is operated (see, e.g., Unexamined Japanese Patent Publication No. H06-61985 (paragraph no. [0007], FIG. 1)), a momentary interruption occurs at the time of switching control modes, making the operation of the heater 601 unstable.
Thus, by using a plurality of controllers with an identical function to simultaneously control a controlled system, it is possible to continue the control action even if any controller fails, thereby enhancing availability.
In the conventional feedback control device in which the controlled system is controlled simultaneously by the multiple controllers, however, equal control currents flow from the individual controllers to the controlled system, so that the loads on the circuits are also the same. Accordingly, in cases where a controller fails or is detached, the control current and the circuit load observed by the normal controller undergo fluctuation. Since it takes time to converge the control current and the circuit load, the control action becomes unstable and high-accuracy, stable control cannot be resumed until the fluctuation ceases.